<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    function f(score) {
        if (score<0||score>100){
            return 无效分数;
        }
        if (score===100){
            return 'A';
        }
        if (score>=80 && score<100){
            return 'B';
        }
        if (score==60){
            return 'C';
        }
        if (score>0 && score<60){
            return 'D';
        }
    }
    var a = f(55);
    console.log(a);
</script>

</body>
</html>